Afrocymbella nyassae, Stone & Wilson & Jovanovska, 2022

Stone, Jeffery R., Wilson, Mallory C. & Jovanovska, Elena, 2022, The genus Afrocymbella (Bacillariophyceae) from lakes Malawi and Tanganyika, with description of new fossil and extant species, Phytotaxa 556 (1), pp. 21-48 : 25

Afrocymbella nyassae sp. nov. (LM Figs 32–53 , SEM Figs 54–59 )

Description:— Valve length 20–30 μm, valve breadth 7–10 μm. Length to breadth ratio: 2.8. Valves distinctly dorsiventral and heteropolar, semirhomboid-lanceolate ( Figs 32–59 ). Dorsal margin distinctly arched, ventral margin with gibbous center portion, though otherwise slightly convex ( Fig. 55 ). Valve apices with rostrate and nearly symmetrical poles ( Figs 32–53 ). Internally, valve apices bear small to indistinguishable pseudosepta ( Fig. 59 ). APF of round poroids present at footpole ( Fig. 56 ), surrounded internally by knobby jointed ridge of silica (white arrow in Fig. 59 ). Axial area narrow, dorsal margin slightly curved ( Figs 32–53 , 55 ). Central area round, slightly asymmetric, wider on dorsal side ( Figs 32–53 , 54 ). Single stigmoid present on dorsal side of central nodule, externally circular to transapically elongated ( Figs 54, 55 ), internally elongated slit ( Figs 57, 58 ). Raphe curved and slightly sinusoidal ( Figs 54, 55 ). External proximal raphe ends expanded into small pores dorsally deflected ( Figs 54, 55 ), internally covered by nodular or flap-like silica development ( Figs 57, 58 ). External distal raphe terminal fissures ventrally deflected ( Figs 55, 56 ), internally slightly elevated into small lobed helictoglossa at some distance from valve apices ( Figs 57, 59 ). Striae radiate throughout, uninterruptedly continuing onto valve mantle ( Fig. 55 ), 14–16 in 10 μm at middle; 18 in 10 μm at valve apices. Striae composed of 30–35 lineate to slit-like areolae in 10 μm, internally ellipsoid without velum ( Fig. 57 ).

Type:— REPUBLIC OF MALAWI. Lake Malawi central basin, Core MAL05-1 C, Drive 26E2; mud, sampled at 17–18 cm; composite 83 m depth (corresponding to ~135.5 ka), 11°17’39.6” S 34°26’09.0” E, J. R. Stone, 22 nd May 2006 (holotype designated here, circled specimen ANS-GC17194! = Fig. 39 , isotypes circled specimens ANSGC17195 !, BM-101978! Cleaned type materials: ANS-GCM 33026.

Registration: https://zoobank.org/NomenclaturalActs/13f4f4e3-74e6-41ba-bd38-a84665cfacd3

Etymology:— The species name ‘ nyassae ’ refers to the type locality, Lake Malawi (aka Lake Nyassa).

Distribution:— To date, Afrocymbella nyassae sp. nov. has only been observed from sediment assemblages of Lake Malawi ranging from ~140 ka to 90 ka in the MAL05-1C drillcore (1C, Fig. 1 ). It is likely an extinct species, seemingly without extant populations.

Figures

FIGURE 1. Map of the lakes Tanganyika and Malawi, showing the sampling locations of modern (red circles) and fossil (cores MAL05- 1C and NPO4-KH1-1K; white circles) specimens used in this study.

FIGURES 32–53. Afrocymbella nyassae sp. nov., LM valve views.All valves displayed from the type material (core sample MAL05-1C- 26E2, 17–18 cm, ~135.5 ka). Fig. 39. Holotype specimen. Scale bar = 10 μm.

FIGURES 54–56. Afrocymbella nyassae sp. nov., SEM external valve views. 54. Half-valve showing uniseriate striae and single stigmoid positioned distally from the round, slightly asymmetric central area. 55. View of the whole valve showing raphe structure and striae continuing onto valve mantle. 56. Close view of distal raphe terminal fissure andAPF of round poroids. Scale bars = 5 μm (Fig. 54), 2 μm (Fig. 55), 1 μm (Fig. 56).

FIGURES 57–59. Afrocymbella nyassae sp. nov., SEM internal valve views. 57. View of the whole valve showing raphe structure and areolae without velum. 58. Detail of mid-valve of a broken valve showing proximal raphe ends covered by flap-like silica development and antler-like intermissio stigmoid. 59. Close view of valve apices showing APF with knobby jointed ridge of silica (white arrow) and distal raphe fissures slightly raised into small helictoglossae. Scale bars = 5 μm (Figs 57–58), 2 μm (Fig. 59).